Method and solution for gold electroplating



United States Patent 3,505,182 METHOD AND SOLUTION FOR GOLD ELECTROPLATING Donald S. Pokras, Richardson, Tex., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware No Drawing. Filed Oct. 1, 1965, Ser. No. 492,336 Int. Cl. C23b /28, 5/46 US. Cl. 204-46 4 Claims ABSTRACT OF THE DISCLOSURE Disclosed is a gold electroplating solution and method for its use. The solution consists essentially of a solution of alkali metal gold cyanide and an alkali metal pyrophosphate. The solution can be utilized within the pH range of 5.5 to 6.5 to produce a thin plating which meets semiconductor alloying and heat test requirements which require much thicker plating using prior art methods. The solution can also be used with benzyl alcohol within the pH range of 7.0 to 8.0 to produce a bright gold plate.

This invention relates to gold electroplating.

In many instances it is desired to provide a thin gold plate on another metal, e.g., on copper, nickel, Kovar and the like.

The gold-plated product is often used at relatively high temperatures. Thus it is necessary that the thickness and continuity of plating be such that relatively rigid heat tests can be passed.

In other instances, a gold-plated product is alloyed to a semiconductor such as germanium, silicon, and the like. This practice, which finds application in the semiconductor and device arts, has in the past required a minimum of from about 80 to 100 micro-inch thickness of gold plate to properly accomplish the alloying.

The present invention provides a gold electroplating solution and method for its use by which a high quality film of gold may be electrodeposited. The film produced by the present invention need be present in thickness of only about micro-inch in order to pass comparable heat tests that previously required thicknesses of 80 to 100 micro-inch of plate, using prior art technology. Moreover, a film of only 35 micro-inch will alloy with semiconductor material, e.g., silicon and germanium.

By addition of a brightening agent and adjustment of pH, the electroplating solution of the present invention can be utilized to apply a substantially 24-carat bright gold plate.

For many years, the most common gold plating solution was of the cyanide electrolyte type. Such solution utilizes sodium gold cyanide or potassium gold cyanide as a source of gold, and either sodium or potassium cyanide as an electrolyte. Commonly, sodium or potassium phosphate is present as a grain refiner in such solution. The pH of this type solution ranges from about 11 to 12.

In the last several years, so-called acid gold plating solutions have been developed. These are aqueous solutions of acid salts and organic acids, such as tartrates and citrates. They also contain either sodium or potassium gold cyanide. A common pH range for these acid gold plating solutions is about 3.5 to 4.0.

A relatively recent development, akin to the acid gold solution type, involves the use of certain phosphates or phosphites as electrolyte. In combination with such electrolyte, hydrazine or substituted hydrazine is utilized. For example, see United States Patent 3,156,635 involving this system.

The various prior art baths or solutions referred to above are good for certain specific purposes; however, for

providing a thin quality film to pass high heat tests that is suitable for alloying with semiconductor materials, both the basic and acid gold plating solutions have not proved entirely satisfactory.

It is an object of the present invention to provide a new and improved plating solution for gold which is superior to prior art solutions for certain purposes.

Moreover, it is an object of the present invention to provide such a solution by which a comparatively thin coat of gold plate may be applied to accomplish what by prior art technology required a much thicker coat.

A further object of the present invention is to provide a versatile plating solution which achieves the foregoing objects within certain pH ranges yet which can be simply adjusted by raising pH and adding a small amount of brightening agent to make the plating solution capable of providing substantially 24-carat bright gold plate.

Generally speaking, the present invention provides a gold electroplating solution which has as its electrolyte a solution of alkali metal pyrophosphate. The preferred pyrophosphate is potassium.

The gold electroplating solution of the present invention consists essentially of a solution of alkali metal gold cyanide and alkali metal pyrophosphate, and is preferably utilized within the pH range of 5.5 to 6.5 or 7.0 to 8.0, depending upon the desired type of plating to be achieved. It a thin plate for purposes of alloying and passing heat test is desired, the lower pH will be selected. If a bright gold plate is desired, a brightening agent is added to the solution and pH is adjusted to a higher value.

For thin plating which alloys well and yet passes heat tests, the pH range preferred is about 5.5 to 6.5. The pH is preferably adjusted with phosphoric acid plus phosphorous pentoxide, which in effect provides phosphoric acid content.

For bright plating, a brightening agent (specifically an aryl alkyl alcohol such as benzyl alcohol) is added to the pyrophosphate electrolyte-gold solution in minor proportions. In this instance, the pH is adjusted to between about 7 and 8 with ammonium hydroxide.

At whatever pH range the present invention may be practiced, the concentration of alkali metal pyrophosphate is preferably such that on the order of about one lb./ gallon of pyrophosphate is provided. Best results appear to be achieved when the alkali metal pyrophosphate concentration is within about 10% of such concentration.

Preferably the gold content provided by the alkali metal gold is such that on the order of about one oz. troy gold/ gallon of solution is provided.

The following specific examples are ofiered by way of illustration of the present invention and are not intended to be taken as limiting its scope.

EXAMPLE 1 A solution having the following composition is prepared:

Potassium gold cyanidel.49 troy oz./gallon Potassium pyrophosphate-2.00 lbs./ gallon The pH of the foregoing solution is adjusted to 6.5 with so-called 105% phosphoric acid (phosphoric acid plus phosphorus pentoxide, known in the trade as Phospholeum).

The resulting solution is seen to have a gold concentration of approximately one oz. gold/gallon. Its specific gravity is about 22-26 Baum.

This solution is brought to a temperature of about F. and electroplating is conducted, using a platinized titanium anode and a nickel plated stainless steel article as the cathode. Current density is approximately one ampere/sq. ft.

Thirty-five micro-inch of gold plate is deposited on the nickel plated stainless steel article. The plate is found to be of high enough quality to pass a heat test which consists of baking in air at 500 C. for a period of 20 minutes. Moreover, the resulting plate is found to alloy well with germanium and silicon.

EXAMPLE 2 The preceding example is repeated, however, sodium gold cyanide in quantity of 1.38 troy oz./ gallon is used in place of the potassium gold cyanide employed therein.

Plating is conducted in accordance with the same set of conditions, except that a high purity carbon anode is used in place of the platinized titanium anode therein employed. Substantially the same results are obtained, with a 35 micro-inch thickness plate being deposited which passes the heat test referred to above and alloys satisfactorily with germanium and silicon.

EXAMPLE 3 Example 1 is repeated, but at a pH of 6.5 and a current density of 0.75 ampere/ sq. ft. Potassium gold cyanide is employed as the gold-bearing material in this particular instance.

A layer of 35 micro-inch plate in accordance with this example passes both the heat test and alloying requirements.

EXAMPLE 4 The foregoing example is repeated, however pH is approximately 5.5 (as adjusted with 105% phosphoric acid) and current density about 1.25 ampere/sq. ft. Substantially the same results are obtained, with plating of the same type and general quality.

EXAMPLE 5 The solution set forth in Example 1 has added to it a brightening agent in minor proportions. The agent employed in this particular example is benzyl alcohol, in quantity of 0.05% by weight, based on the total solution. The pH of the plating solution is adjusted to 7.5 by addition of ammonium hydroxide. Plating with the same conditions as those described in Example 1 provides 24'carat bright plate.

EXAMPLE 6 The preceding example is repeated, however, sodium pyrophosphate is substituted for potassium pyrophosphate. Moreover, equal quantities of methanol, ethanol, n-propanol, tertiary butanol and benzyl alcohol are added until a combined additive quantity of 0.07% by Weight, based on the weight of solution, is obtained, Upon plating, the results obtained are substantially the same as those described above.

Lithium pyrophosphate and other alkali metal pyrophosphates may be utilized in the present invention, but potassium and sodium pyrophosphate, respectively, are presently thought best.

While potassium and sodium gold cyanide are the preferred gold-carrying agents, others might be utilized, e.g., lithium or cesium gold cyanide or other alkali metal gold cyanides.

It is seen that the present invention provides a plating solution which is quite versatile in that it may be utilized at two different pH ranges, both close to neutral, contrasted to relatively high and low pH prior art solutions. At the lower pH range, between about 5.5 and 6.5, an extremely thin film may be obtained which is adequate for alloying with semiconductor material. Such film is also adequate to pass rigorous heat tests. At the higher pH of about 7 to 8, with the addition of a small amount of brightening agent, a bright, substantially 24-carat plate is produced.

The present invention utilizes an alkali metal pyrophosphate as an electrolyte. This is to be contrasted to the normally employed cyanide electrolyte of the prior art and certain organic acid salts. Moreover, the pyrophosphate of the present invention is to be contrasted to prior art technology involving ordinary phosphates and phosphites.

What is claimed is:

1. A gold electroplating aqueous solution consisting essentially of a solution of alkali metal gold cyanide in sufiicient quantity to provide about one ounce troy of gold per gallon of solution, and alkali metal pyrophosphate in sufi'icient quantity to provide, on a pyrophosphate basis, about one pound of pyrophosphate per gal- 1011 of solution, said solution having a pH from about 7 to 8 and containing benzyl alcohol as a brightening agent in sufiicient quantity to produce a substantially 24 carat gold plate.

2. The electroplating solution of claim 1 in Which said alkali metal pyrophosphate is potassium pyr'ophosphatc, the concentration thereof being about two pounds per gallon of solution.

3. In the method of electroplating an article with gold, the step which comprises passing a current from an inactive anode through the gold electroplating solution of claim 1 in which said article serves as the cathode, and wherein said alkali metal pyrophosphate is potassium pyrophosphate.

4. In the method of electroplating an article with gold, the step which comprises passing a current from an inactive anode through the gold electroplating solution of claim 1 in which said article serves as the cathode, and wherein said alkali metal pyrophosphate is sodium pyrophosphate.

References Cited UNITED STATES PATENTS 3,193,350 7/ 1965 Beltz et al. 23-165 2,871,171 1/ 1959 Atkinson 204-52 XR 923,864 6/ 1909 Levy 20446 XR 2,812,299 11/ 1957 Volk 204--46 XR 3,039,942 6/1962 Cox 20446 XR 3,057,789 10/1962 Smith 204-46 FOREIGN PATENTS 695,628 10/ 1964 Canada. 1,314,288 11/ 1962 France.

354,643 7/ 1961 Switzerland.

JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner 

